[func.wrap.ref.class] Add expos-only members explicitly

Author: tkoeppe

source/utilities.tex

@@ -13493,7 +13493,10 @@
 
   private:
     template<class... T>
-      static constexpr bool @\exposid{is-invocable-using}@ = @\seebelow@;     // \expos
+      static constexpr bool @\exposidnc{is-invocable-using}@ = @\seebelownc@;     // \expos
+
+    R (*@\exposidnc{thunk-ptr}@)(@\exposidnc{BoundEntityType}@, Args&&...) noexcept(@\placeholdernc{noex}@);  // \expos
+    @\exposidnc{BoundEntityType}@ @\exposidnc{bound-entity}@;                               // \expos
   };
 
   // \ref{func.wrap.ref.deduct}, deduction guides
@@ -13509,17 +13512,14 @@
 \pnum
 An object of class
 \tcode{function_ref<R(Args...) \cv{} noexcept(\placeholder{noex})>}
-stores
-%FIXME: What is "thunk-ptr"? where did it come from?
-a pointer to function \exposid{thunk-ptr} and
-%FIXME: What is "bound-entity"? where did it come from?
+stores a pointer to function \exposid{thunk-ptr} and
 an object \exposid{bound-entity}.
 \exposid{bound-entity} has
-an unspecified trivially copyable type \tcode{BoundEntityType}, that
+an unspecified trivially copyable type \exposid{BoundEntityType}, that
 models \libconcept{copyable} and
 is capable of storing a pointer to object value or a pointer to function value.
 The type of \exposid{thunk-ptr} is
-\tcode{R(*)(BoundEntityType, Args\&\&...) noexcept(\placeholder{noex})}.
+\tcode{R(*)(\exposidnc{BoundEntityType}, Args\&\&...) noexcept(\placeholder{noex})}.
 
 \pnum
 Each specialization of \tcode{function_ref} is